Hydrogels for aqueous lithium/air battery cells

ABSTRACT

Li/air battery cells are configurable to achieve very high energy density. The cells include a protected a lithium metal or alloy anode and an aqueous catholyte in a cathode compartment. In addition to the aqueous catholyte, components of the cathode compartment include an air cathode (e.g., oxygen electrode) and a variety of other possible elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/159,786 filed Mar. 12, 2009, titled HIGH ENERGY DENSITY AQUEOUSLITHIUM/AIR CELLS; and U.S. Provisional Patent Application No.61/078,294 filed Jul. 3, 2008, titled AQUEOUS LI/AIR CELLS; and U.S.Provisional Patent Application No. 61/061,972 filed Jun. 16, 2008,titled AQUEOUS LI/AIR CELLS. Each of these prior applications isincorporated herein by reference in its entirety and for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to active metal electrochemicaldevices. More particularly, this invention relates to Li/air batterycells capable of achieving high energy densities

The large free energy for the reaction of lithium with oxygen hasattracted the interest of battery researchers for decades. Thetheoretical specific energy for lithium/air chemistry far exceeds Li-ionbattery chemistry. The high specific energy for metal/air chemistrieshas long been recognized, as evidenced by the development and commercialsuccess of the Zn/Air battery. However, Li/air chemistry introduces somerather unique challenges.

Lithium is a reactive alkali metal and is incompatible with aqueouselectrolytes. Corrosion of the negative electrode due to reaction of themetal anode with water and oxygen in aqueous electrolyte is clearly amuch more serious issue for lithium than it is for zinc. The corrosionrate of lithium metal in aqueous electrolytes is on the order of severalamps/cm² for neutral electrolytes and drops to tens of mA/cm² in highlybasic media. Accordingly, with the exception of some early work byLittauer on lithium/water batteries, the historical development oflithium/air batteries has been dominated by the use of aproticnon-aqueous electrolytes.

Essential to the development of aqueous Li/air batteries is the abilityto stabilize the lithium anode in the presence of water and oxygen. U.S.Pat. Nos. 7,282,296; 7,282,302; and 7,282,295; and U.S. PatentApplication Pub. No.: US 2004/0197641 to Visco et al., discloseprotected lithium anodes having protective membranes and protectivemembrane architectures that are stable in water environments and arecapable of discharging into aqueous electrolytes.

The present invention is directed to the furtherer development andenhancement of the overall performance of Li/air battery cells via thechemistry taking place at the cathode.

SUMMARY OF THE INVENTION

The present invention relates to Li/air battery cells. The cells includea protected lithium electrode and an aqueous catholyte in a cathodecompartment. The cells are configurable to achieve high energydensities, in accordance with various aspects of the invention. Inaddition to the aqueous catholyte, which is defined as electrolyte incontact with the cathode, components of the cathode compartment includean air cathode (e.g., oxygen electrode) for the reduction of molecularoxygen

In various aspects the invention relates to the chemistry of the subjectlithium/air cells, and in particular to that chemistry which occurs inthe cathode compartment when it (the compartment) is exposed to ambientair during cell operation.

In accordance with Li/air cells of the current invention and the aboveaspect, in various embodiments active species dissolved in the catholytepartake in the cell reaction, and discharge products form that renderthe cathode compartment hygroscopic. By this expedient, the cathodecompartment scavenges water from the ambient air during cell operation,and this facilitates a number of advantages, including: preventing dryout of the catholyte, prolonging the discharge, and enhancing the cellenergy density (Wh/kg and Wh/l).

In accordance with the instant invention, the aqueous catholyte isactive in that species dissolved therein participate, as a reagent, inthe cell discharge reaction.

In various embodiments, the active catholyte comprises water and anactive substance material dissolved therein, and the dissolutiongenerates active species that participate, as a reagent, in the celldischarge reaction. Typically, the active substance material is anactive salt or a solid active compound. However, the invention is notlimited as such, and in certain embodiments the active substancematerial is an active liquid, including inorganic liquids.

Appropriate concentrations of active and non-active salts may impart anumber of benefits to the cell, including: i) rendering the cathodecompartment hygroscopic during cell operation; ii) effectuating, priorto initial discharge, a very low vapor pressure of water above thecatholyte; iii) enhancing or maintaining the conductivity of thecatholyte at various stages of discharge, and iv) stabilizing theprotective membrane/catholyte interface during storage under opencircuit conditions.

In various embodiments, high concentrations of dissolved active saltsare desirable, as this may render the compartment, prior to initialdischarge, hygroscopic and, during discharge, may effect a lowering ofthe catholyte's equilibrium relative humidity (ERH) sufficient toprovide a driving force for the ingress of water into the cathodecompartment from the ambient air. In specific embodiments, active saltconcentrations of at least 1M, or at least 2M, or at least 3M, or atleast 4M; for example about 2M or about 3M or about 4M, are suitable. Incertain embodiments, catholyte saturated or nearly saturated withdissolved active salts can be used, and is preferred.

In various embodiments, the cathode compartment, prior to initialdischarge, contains a water soluble solid phase active salt thatoperably dissolves in contact with the catholyte, and thereby, in thatprocess, generates dissolved active salt species that participate, as areagent, in the cell reaction. Accordingly, solid phase active saltsprovide a source from which a relatively large amount of dissolvedactive salt species may form in the catholyte during cell operation. Byuse of the term “operably dissolves” or “operable dissolution” it ismeant that the referenced material dissolves during cell operation.

In various embodiments the catholyte comprises at least two differentdissolved salts. For instance: an active first salt and a differentsecond salt. The salts (first or second) may be active or non-active. Invarious embodiments the first salt is active and the second salt,non-active, is a lithium salt, preferably a hygroscopic lithium saltsuch as, but not limited to, halides, including lithium bromide (e.g.,LiBr), lithium iodide (e.g., LiI), and lithium chloride (e.g., LiCl) andnitrates (e.g., LiNO₃)).

In various embodiments the catholyte, prior to initial discharge, has alow equilibrium relative humidity (ERH). Preferably the ERH of thecatholyte, prior to initial discharge or prior to cell activation, isless than 50%, less than 40%, less than 30%, less than 20% or less than15%, at room temperature (about 20° C.). And preferably, it is lowenough to render the cathode compartment hygroscopic. Low ERH values maybe achieved through the use of high dissolved salt concentrations in thecatholyte (including combinations of active and non-active salts) orsaturating the catholyte with salt, particularly hygroscopic salts, andit may be achieved by introducing solid phase salts into the cathodecompartment, including solid phase active salts and solid phasenon-active salts, or combinations thereof. In specific embodimentscatholyte ERH is lowered by a combination of dissolved active andnon-active salts. For instance, a high concentration of a dissolvedactive salt (for example, at least 3M (e.g., about 4M) and a lowerconcentration of a dissolved lithium salt (for example, at least 2M;e.g., 2M)

In specific embodiments lithium salts are dissolved in the catholyte ina sufficient concentration to prevent resistance rise in the cell duringstorage under open circuit conditions prior to initial discharge. Inthis regard, initial (prior to initial discharge) Li salt (ion)concentrations of at least 2M, for example 2M, can be effectively used.

In various embodiments the Li/air cell of the instant invention isdischarged over a first stage that corresponds to dissolved active saltspecies participating, as a reagent, in the discharge reaction, and asecond stage of discharge in which dissolved active salt species do notparticipate. Typically, water molecules are generated during the firststage of discharge and consumed, as a reagent, during the second stage.

In various embodiments, the discharge product, hygroscopic, may be usedto facilitate water management in the cathode compartment, includingpreventing catholyte dry out. Moreover, in various embodiments, theabsorptive capacity of the cathode compartment enables the cells of theinstant invention to be manufactured with less water than that which isnecessary for the cell to deliver its rated capacity. In accordance withthese embodiments, the cell, initially having an insufficient amount ofwater, absorbs the necessary amount of water from the ambient air duringcell operation.

In another aspect, the invention provides a Li/air cell comprising areservoir structure incorporated in the cathode compartment, and whichmay serve several functions, including: providing a porous physicalstructure for catholyte and, when present, solid phase active salts;retaining water operably absorbed from the ambient air; andaccommodating both liquid and solid products of discharge. In variousembodiments, the reservoir, in the form of a layer, may have a porousmetal oxide structure, or a carbonaceous structure, or a polymericstructure that is sufficiently elastic to expand during discharge. Incertain embodiments, in order to tailor the location of where soliddischarge products form, the compartment may comprise more than onereservoir layer, e.g., a first reservoir layer adjacent the cathode anda second reservoir layer adjacent the protected anode.

In yet another aspect, the invention provides a Li/air cell comprising ahydrogel or a hydrogel layer which may be utilized to great advantage inthe cell, including improving specific energy of the cell by allowing ahigh loading of active and supporting electrolyte salts, both of whichmay be dissolved in the catholyte or present in the form of un-dissolvedsolids (e.g., active solid phase salts and solid supporting salt (e.g.,lithium salts including LiCl, LiBr and LiI). In various embodiments, thehydrogels are disposed in the cathode compartment between the anode andthe cathode, and their swelling properties make them particularlysuitable for retaining large amounts of water absorbed by the catholyteduring discharge, and by this expedient preventing catholyte leakage.

In yet even another aspect, the invention provides a Li/air cell havingan inventive air cathode capable of accommodating large amounts ofdischarge product. Another novel feature of the inventive air cathode isthat the active carbon layer is disposed in the bulk of the cathode andthe cathode expands upon discharge. By this expedient, the inventive aircathodes are particularly suitable for high capacity Li/Air cellsbecause the cathode, expanding on discharge, will continue toaccommodate large amounts of solid product as it forms.

The present invention provides a lithium/air cell comprising a protectedlithium electrode (PLE) and a cathode compartment comprising a cathode(e.g., an air cathode) for the reduction of molecular oxygen and anaqueous catholyte, which is defined herein as an aqueous electrolytesolution in contact with the cathode. In accordance with the instantinvention, the aqueous catholyte is active in that it or itsconstituents (e.g., dissolved active salts and/or water) partake in thecell reaction during discharge.

In various embodiments of the invention, the battery cells also compriseone or more of the following: prior to initial discharge, solid phaseactive salt in the cathode compartment; a catholyte saturated, or nearlysaturated, with active salt or non-active salt or a lithium compound orsome combination thereof; a catholyte comprising a combination of atleast two different dissolved salts, including a high concentration of afirst active salt and a lower concentration of a second hygroscopiclithium salt (e.g., LiCl); a catholyte comprising a metal halide activesalt wherein said metal of the active salt is not lithium; a catholytecomprising a nitrate active salt (e.g., ammonium nitrate); a catholytecomprising a supporting lithium salt concentration of 2 molar Li orhigher or otherwise sufficient to prevent resistance rise in the cellduring storage under open circuit conditions prior to initial discharge;a combination of dissolved active and supporting salts in the catholyte;a porous inorganic solid reservoir structure; a solid reservoirstructure comprising a carbonaceous porous structure; a solid reservoirstructure configured to expand upon discharge; a hydrogel reservoirstructure; a cathode pore structure configured to accommodate insolubledischarge products such that operation of the cell is not disruptedprior to substantially complete discharge; a cathode configured toexpand upon accommodating discharge product.

The protected anode comprises a lithium ion conductive protectivemembrane having a first and second surface. The membrane is impermeableto liquids and air and is configured to prevent direct contact betweenactive lithium and constituents of the cathode compartment; particularlyit protects the lithium anode from contacting aqueous catholyte andexposure to ambient air. The first surface of the membrane faces thelithium anode and the second membrane surface faces the cathodecompartment. In various embodiments, the catholyte, in contact with thecathode, also contacts and substantially covers at least a portion ofthe protective membrane second surface; and in contact, the protectivemembrane is chemically compatible with the aqueous catholyte.

The cathode compartment, prior to initial discharge, comprises anaqueous catholyte comprising water and a salt dissolved therein. Thecatholyte, in contact with the protective membrane and the cathode,provides an ionically conductive medium of sufficient ionic conductivityto support the electrical current that flows, during discharge, betweenanode and cathode. And when the cell is a secondary, the conductivity issufficient to support the charging current.

In various embodiments the catholyte salt, active or non-active,comprises halogen (e.g., chlorine, bromine, or iodine) or nitrate orammonium. In certain embodiments the active salt is a nitrate (e.g.,NH₄NO₃) or a halide or an ammonium compound or it is a compound thatdissolves, in the catholyte, hydrolytically (e.g., a compound comprisinga metal (e.g., AlCl₃). In specific embodiments the halide salt is anammonium halide salt (NH₄Br, NH₄Cl, NH₄I) or a metal halide salt (e.g.,MgCl₂), wherein the metal of the halide is not lithium, for example themetal may be aluminum or titanium, or more generally an alkaline metalor a transition metal. Typically, when the active salt is a metalhalide, it does not contain lithium.

In various embodiments, the catholyte, prior to initial discharge,comprises more than one dissolved salt. For instance, the catholyte maycomprise more than one active salt, or a combination of active salts andnon-active salts (e.g., lithium salts). Or the catholyte may comprisetwo or more active salts and, optionally, at least one supportingelectrolyte salt dissolved therein. In a specific embodiment thecatholyte comprises two different lithium salts: a first lithium salt(LiCl) which is very hygroscopic and soluble and a second lithium salt(LiBr) that while significantly heavier than the first salt is extremelyhygroscopic and so its facility to drive water absorption into thecompartment offsets the additional weight of its anion, and the combinedsalt system is an effective mechanism for balancing these effects.

In specific embodiments, the catholyte comprises an active salt and anon-active salt, the active salt having a higher concentration than thatof the non-active salt; e.g., a high concentration of an active salt anda lower concentration of lithium salt. In certain embodiments, thecatholyte comprises, dissolved therein, more than one type of activesalt and more than one type of lithium salt.

In various embodiments the aqueous catholyte comprises at least onedissolved active salt (e.g., NH₄Cl) that participates in the cell orcathode reaction during discharge to form corresponding lithium salts(e.g., LiCl), and in some instances the cell reaction generates water.

In certain embodiments, the lithium salt that forms during discharge ishighly soluble in the catholyte or is hygroscopic and absorbs moisturefrom the ambient air, or is both highly soluble and hygroscopic. Thelithium salt formed as a result of the cell discharge reaction may be insolution (i.e., a dissolved salt) or it may precipitate out of solutionas a solid. In various embodiments, the corresponding lithium saltformed as a result of the discharge reaction is highly soluble and hassignificantly higher solubility in water than the dissolved active salt.

In various embodiments the catholyte comprises a dissolved active salt.In certain embodiments the dissolved active salt is a weak acid. Incertain embodiments the weak acid salt is dissolved in the catholyte inconcentrations of about 2 molar or greater.

In certain embodiments the dissolved active salt is a halide salt, suchas an ammonium halide salt (e.g., NH₄Cl, NH₄Br, and NH₄I) or a metalhalide salt (e.g., AlCl₃). In certain embodiments the dissolved activesalt is a nitrate salt, e.g., NH₄NO₃, or ammonium thiocynate (i.e.,NH₄CNS).

In various embodiments, prior to initial discharge, an active solidphase salt is present in the cathode compartment, typically in thereservoir, which reacts (or otherwise interacts) in contact with waterof the cathode compartment or reacts in contact with aqueous catholyteto form a dissolved active salt in the catholyte. In various embodimentsthe solid phase active salt is deliquescent. In various embodiments theactive solid substance is microencapsulated. In various embodiments thesolid phase active salt comprises one or more of halogen (e.g.,chlorine, bromine, iodine) or nitrate, or ammonium. In certainembodiments the solid phase active salt is a halide salt. In certainembodiments thereof it is an ammonium halide salt (e.g., NH₄Cl, NH₄Br orNH₄I). In specific embodiments the solid phase active salt is a nitrate(e.g., NH₄NO₃). In other embodiments thereof it is a metal halide salt,where the metal is not lithium. In specific embodiments thereof themetal halide salt does not comprise lithium metal (e.g., AlCl₃). Inspecific embodiments the solid phase active salt is a metal compoundthat dissolves hydrolytically in the catholyte, and the metal of thecompound is not lithium. In various embodiments, prior to initialdischarge, the catholyte is a saturated salt solution in contact withsolid phase active salt. The saturated salt solution may be saturatedwith at least one of the following: an active salt or a non-active salt,or a lithium salt, or a combination thereof.

During cell discharge species are formed in the catholyte or precipitateout of the catholyte as a solid, and these species are generallyreferred to herein as discharge products. In various embodiments, thedischarge product is hygroscopic and sufficiently lowers the water vaporpressure above the catholyte to render or maintain the cathodecompartment hygroscopic, or otherwise in equilibrium with the ambientrelative humidity.

In various embodiments, at least one of the discharge products is alithium compound, typically a lithium salt, composed of lithiumcation(s) and an anion of the active salt. Accordingly, in someinstances, the discharge product may be considered, and is sometimesreferred to, herein, as a corresponding lithium salt, which is to meanthat the discharge product is a species formed between lithium ions(e.g., those which pass into the catholyte from the anode duringdischarge) and an anion of the active salt. For instance, when theactive salt is a halide (e.g., NH₄Cl or NH₄Br), the discharge productmay be a lithium halide salt, (e.g., LiCl or LiBr).

The invention also provides a variety of cell fabrication techniques andconfigurations.

These and other features of the invention are further described andexemplified in the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of a Li/air battery cell inaccordance with the present invention.

FIG. 1B shows a Li/air battery cell in accordance with the presentinvention enclosed in a cell case in cross sectional and perspectivedepictions.

FIGS. 2A-D illustrate various alternative configurations of a protectivemembrane architecture in accordance with the present invention.

FIG. 3 illustrates performance of a Li/air cell having 4M NH₄Cl, 2M LiClcatholyte and zirconia felt reservoir layers.

FIG. 4 illustrates performance of a Li/Air cell having 4M NH₄Cl, 2M LiClcatholyte and graphite felt reservoir layers.

FIG. 5 illustrates stability of a solid electrolyte protective membranein contact with 4M NH₄Cl, 2M LiCl catholyte during long-term storage.

FIG. 6 illustrates comparative performances of Li/Air cells having 4MNH₄NO₃, 2M LiNO₃ and 1M LiOH catholytes.

FIG. 7 illustrates performance of a Li/Air cell with 2.7M AlCl₃, 1M LiClcatholyte and alumina felt reservoir layers.

FIG. 8 illustrates performance of a Li/Air cell with 2.7M AlCl₃, 1M LiClcatholyte and graphite felt reservoir layers.

FIG. 9 illustrates performance of a Li/Air cell having alumina feltreservoir layers impregnated with solid NH₄Cl salt and additionallyfilled with 4M NH₄Cl, 2M LiCl catholyte.

FIG. 10 illustrates performance of a Li/Air cell having pressed pelletsof NH₄Cl salt mixed with alumina powder and Whatman micro fiber filtersGF/A as second reservoir layers filled with 4M NH₄Cl, 2M LiCl catholyte.

FIG. 11 illustrates performance of a Li/Air cell having crosslinkedpolyacrylamide hydrogel reservoir layers containing dissolved 4M NH₄Cland 2M LiCl.

FIG. 12 illustrates performance of a Li/Air cell having crosslinkedpolyacrylamide hydrogel reservoir layers containing dissolved 2M LiCland loaded with solid NH₄Cl.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to specific embodiments of theinvention. Examples of the specific embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these specific embodiments, it will be understood thatit is not intended to limit the invention to such specific embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. The present invention may be practiced without some or all ofthese specific details. In other instances, well known processoperations have not been described in detail so as to not unnecessarilyobscure the present invention.

INTRODUCTION

The present invention relates to Li/air battery cells. In various ways,cells in accordance with the present invention are configurable toachieve very high energy density, for example, higher than would beotherwise attainable without active salt in the catholyte. The cellsinclude a protected lithium electrode (e.g., protected lithium metal oralloy or intercalation anode) and an aqueous catholyte in a cathodecompartment. In addition to the aqueous catholyte, which is defined aselectrolyte in contact with the cathode, components of the cathodecompartment include an air cathode (e.g., oxygen electrode) for thereduction of molecular oxygen.

In various embodiments of the invention, the battery cells also compriseone or more of the following: prior to initial discharge or prior toinitial activation of the cell or subsequent to initial discharge, asolid phase active salt in the cathode compartment; a catholytecomprising a saturated salt solution, or a nearly saturated saltsolution; a catholyte comprising a combination of at least two differentdissolved salts, including a high concentration of an active salt and alower concentration of a second hygroscopic supporting salt (e.g., alithium salt); a catholyte comprising a metal halide or ammonium halideactive salt or salts wherein said metal of the active salt is notlithium; a catholyte comprising a nitrate active salt (e.g., ammoniumnitrate); a catholyte comprising a supporting lithium salt concentrationof 2 molar Li or higher or otherwise sufficient to prevent resistancerise in the cell during storage under open circuit conditions prior toinitial discharge; a combination of dissolved active and supportingsalts in the catholyte; a porous inorganic solid reservoir structure; areservoir comprising a carbonaceous porous structure; a solid reservoirstructure configured to expand upon discharge; a hydrogel reservoirstructure; a cathode pore structure configured to accommodate insolubledischarge products such that operation of the cell is not disruptedprior to substantially complete discharge.

DEFINITIONS

To facilitate a better understanding of the present invention, ratherthan to limit its scope, the following definitions are provided:

As used herein the term “active substance material” refers to a materialsuch as a salt that upon dissolving in a solvent of the catholyte,typically water, generates dissolved active species that participate inthe cell discharge reaction. Furthermore, the dissolved active speciesgenerated by the dissolution of the active substance material may bederived from the dissolution of the salt or the salt may dissolvehydrolytically or dissolve and react to form the active catholytespecies.

As used herein the term “solid phase active substance material” refersto a solid material that upon dissolving in a solvent of the catholytegenerates or forms active species, dissolved in the catholyte, thatparticipate, as a reagent, in the cell discharge reaction. And by theuse of the term “solid phase” it is meant to emphasize that the state ofmatter of the solid phase active salt is solid. Typically the solidphase active substance material is a solid phase active salt, or moregenerally a solid phase active compound.

As used herein and in the claims, the term “dissolution” or “dissolves”or “dissolving” is intended to encompass, without limitation, theprocess whereby a solid substance (e.g., a salt) dissolves, with orwithout complete dissociation, or dissolves hydrolytically or upondissolving further reacts, for instance, to form dissolved activespecies in the catholyte.

The use of the term “cell activation” refers to the initial exposure ofthe cathode compartment to the ambient air with the distinct intent ofeither absorbing water moisture from the ambient air and/or molecularoxygen.

When referring to solid phase active salts, the term “operablydissolves” means that the solid phase active salt dissolves into asolvent of the catholyte during cell operation, which includes thatoperational period corresponding to active discharge, when electricalcurrent flows between anode and cathode, and/or that operational periodcorresponding to the cell resting under open circuit conditions,subsequent to cell activation.

Cell Structure

A battery cell in accordance with the current invention is schematicallyshown in FIG. 1A. The cell comprises a Li anode 1. The anode may be Limetal or a Li metal alloy or Li intercalation material (e.g., lithiatedcarbon). In one example, a Li metal foil may be used. Lithium anodes,including intercalation anodes and lithium alloys and lithium metalanodes are well known in the lithium battery art. In preferredembodiments the anode is lithium metal (e.g., in foil or sintered form)and of sufficient thickness (i.e., capacity) to enable the cell toachieve the rated discharge capacity of the cell. The anode may take onany suitable form or construct including a green or sintered compact(such as a wafer or pellet), a sheet, film, or foil, and the anode maybe porous or dense. Without limitation, the lithium anode may have acurrent collector (e.g., copper foil, or suitable expandable metal)pressed or otherwise attached to it in order to enhance the passage ofelectrons between it and the leads of the cell. Without limitation thecell may be anode or cathode limited. When anode limited, the completedischarge (corresponding to rated capacity) will substantially exhaustall the lithium in the anode. When cathode limited, some active lithiumwill remain subsequent to the cell delivering its rated capacity.

The anode is protected with a protective membrane architecturechemically stable to both the anode and the environment of an adjacentcathode compartment (4). The protective membrane architecture typicallycomprises a solid electrolyte protective membrane 2 and an interlayer 3.The protective membrane architecture is in ionic continuity with the Lianode 1 and is configured to selectively transport Li ions into and outof the cathode compartment 4 while providing an impervious barrier tothe environment external to the anode. Protective membrane architecturessuitable for use in the present invention are described in applicants'co-pending published US Applications US 2004/0197641 and US 2005/0175894and their corresponding International Patent Applications WO 2005/038953and WO 2005/083829, respectively, incorporated by reference herein.

With reference to FIG. 1B, there is illustrated (in cross-section (left)and perspective (right)) an embodiment of a lithium air battery cell 10in accordance with the instant invention. The cell is disposed in a case11 (e.g., a metal or polymeric case, including but limited to a heatsealable multilayer laminate used for that purpose). The case comprisesone or more ports 12 for the passage of oxygen and moisture from theambient air. To effectively reach the cathode and the cathodecompartment, as illustrated in FIG. 1B, the case side wall of whichcontains the ports is adjacent to the cathode.

In many, but not necessarily all embodiments, the cell is activated byremoving a barrier material (not shown) which covers the ports toprevent, prior to cell activation, premature or excessive exposure ofthe cathode compartment to ambient air. The cell activated by removingthe barrier material (e.g., by the act of peeling off a tab (barriermaterial layer).

The case may further comprise an additional port (not shown) forintroducing, water or catholyte into the cathode compartment after thecell has been manufactured. Without limitation, the catholyte (or water)may be introduced, prior or subsequent to one or more of the following:cell activation or initial discharge.

FIGS. 2A-D illustrate representative protective membrane architecturesfrom these disclosures suitable for use in the present invention. Theprotective membrane architectures provide a barrier to isolate a Lianode from ambient and/or the cathode side of the cell while allowingfor efficient ion Li metal ion transport into and out of the anode. Thearchitecture may take on several forms. Generally it comprises a solidelectrolyte layer that is substantially impervious, ionically conductiveand chemically compatible with the external ambient (e.g., air or water)or the cathode environment.

Referring to FIG. 2A, the protective membrane architecture can be amonolithic solid electrolyte 202 that provides ionic transport and ischemically stable to both the active metal anode 201 and the externalenvironment. Examples of such materials are Na-β″ alumina, LiHfPO₄ andNASICON, Nasiglass, Li₅La₃Ta₂O₁₂ and Li₅La₃Nb₂O₁₂. Na₅MSi₄O₁₂ (M: rareearth such as Nd, Dy, Gd).

More commonly, the ion membrane architecture is a composite composed ofat least two components of different materials having different chemicalcompatibility requirements, one chemically compatible with the anode,the other chemically compatible with the exterior; generally ambient airor water, and/or battery electrolytes/catholytes. By “chemicalcompatibility” (or “chemically compatible”) it is meant that thereferenced material does not react to form a product that is deleteriousto battery cell operation when contacted with one or more otherreferenced battery cell components or manufacturing, handling, storageor external environmental conditions. The properties of different ionicconductors are combined in a composite material that has the desiredproperties of high overall ionic conductivity and chemical stabilitytowards the anode, the cathode and ambient conditions encountered inbattery manufacturing. The composite is capable of protecting an activemetal anode from deleterious reaction with other battery components orambient conditions while providing a high level of ionic conductivity tofacilitate manufacture and/or enhance performance of a battery cell inwhich the composite is incorporated.

Referring to FIG. 2B, the protective membrane architecture can be acomposite solid electrolyte 210 composed of discrete layers, whereby thefirst material layer 212 (also sometimes referred to herein as“interlayer”) is stable to the active metal anode 201 and the secondmaterial layer 214 is stable to the external environment. Alternatively,referring to FIG. 2C, the protective membrane architecture can be acomposite solid electrolyte 220 composed of the same materials, but witha graded transition between the materials rather than discrete layers.

Generally, the solid state composite protective membrane architectures(described with reference to FIGS. 2B and C) have a first and secondmaterial layer. The first material layer (or first layer material) ofthe composite is ionically conductive, and chemically compatible with anactive metal electrode material. Chemical compatibility in this aspectof the invention refers both to a material that is chemically stable andtherefore substantially unreactive when contacted with an active metalelectrode material. It may also refer to a material that is chemicallystable with air, to facilitate storage and handling, and reactive whencontacted with an active metal electrode material to produce a productthat is chemically stable against the active metal electrode materialand has the desirable ionic conductivity (i.e., a first layer material).Such a reactive material is sometimes referred to as a “precursor”material. The second material layer of the composite is substantiallyimpervious, ionically conductive and chemically compatible with thefirst material. Additional layers are possible to achieve these aims, orotherwise enhance electrode stability or performance. All layers of thecomposite have high ionic conductivity, at least 10⁻⁷ S/cm, generally atleast 10⁻⁶ S/cm, for example at least 10⁻⁵ S/cm to 10⁻⁴ S/cm, and ashigh as 10⁻³ S/cm or higher so that the overall ionic conductivity ofthe multi-layer protective structure is at least 10⁻⁷ S/cm and as highas 10⁻³ S/cm or higher.

A fourth suitable protective membrane architecture is illustrated inFIG. 2D. This architecture is a composite 230 composed of an interlayer232 between the solid electrolyte 234 and the active metal anode 201whereby the interlayer is impregnated with anolyte. Thus, thearchitecture includes an active metal ion conducting separator layerwith a non-aqueous anolyte (i.e., electrolyte about the anode), theseparator layer being chemically compatible with the active metal and incontact with the anode; and a solid electrolyte layer that issubstantially impervious (pinhole- and crack-free) ionically conductivelayer chemically compatible with the separator layer and aqueousenvironments and in contact with the separator layer. The solidelectrolyte layer of this architecture (FIG. 2D) generally shares theproperties of the second material layer for the composite solid statearchitectures (FIGS. 2B and C). Accordingly, the solid electrolyte layerof all three of these architectures will be referred to below as asecond material layer or second layer.

A wide variety of materials may be used in fabricating protectivecomposites in accordance with the present invention, consistent with theprinciples described above. For example, in the solid state embodimentsof FIGS. B and C, the first layer (material component), in contact withthe active metal, may be composed, in whole or in part, of active metalnitrides, active metal phosphides, active metal halides active metalsulfides, active metal phosphorous sulfides, or active metal phosphorusoxynitride-based glass. Specific examples include Li₃N, Li₃P, LiI, LiBr,LiCl, LiF, Li₂S—P₂S₅, Li₂S—P₂S₅—LiI and LiPON. Active metal electrodematerials (e.g., lithium) may be applied to these materials, or they maybe formed in situ by contacting precursors such as metal nitrides, metalphosphides, metal halides, red phosphorus, iodine, nitrogen orphosphorus containing organics and polymers, and the like with lithium.A particularly suitable precursor material is copper nitride (e.g.,Cu₃N). The in situ formation of the first layer may result from anincomplete conversion of the precursors to their lithiated analog.Nevertheless, such incomplete conversions meet the requirements of afirst layer material for a protective composite in accordance with thepresent invention and are therefore within the scope of the invention.

For the anolyte interlayer composite protective architecture embodiment(FIG. 2D), the protective membrane architecture has an active metal ionconducting separator layer chemically compatible with the active metalof the anode and in contact with the anode, the separator layercomprising a non-aqueous anolyte, and a substantially impervious,ionically conductive layer (“second” layer) in contact with theseparator layer, and chemically compatible with the separator layer andwith the exterior of the anode. The separator layer can be composed of asemi-permeable membrane impregnated with an organic anolyte. Forexample, the semi-permeable membrane may be a micro-porous polymer, suchas are available from Celgard, Inc. The organic anolyte may be in theliquid or gel phase. For example, the anolyte may include a solventselected from the group consisting of organic carbonates, ethers,lactones, sulfones, etc, and combinations thereof, such as EC, PC, DEC,DMC, EMC, 1,2-DME or higher glymes, THF, 2MeTHF, sulfolane, andcombinations thereof. 1,3-dioxolane may also be used as an anolytesolvent, particularly but not necessarily when used to enhance thesafety of a cell incorporating the structure. When the anolyte is in thegel phase, gelling agents such as polyvinylidine fluoride (PVdF)compounds, hexafluropropylene-vinylidene fluoride copolymers (PVdf-HFP),polyacrylonitrile compounds, cross-linked polyether compounds,polyalkylene oxide compounds, polyethylene oxide compounds, andcombinations and the like may be added to gel the solvents. Suitableanolytes will, of course, also include active metal salts, such as, inthe case of lithium, for example, LiPF₆, LiBF₄, LiAsF₆, LiSO₃CF₃ orLiN(SO₂C₂F₅)₂. In the case of sodium, suitable anolytes will includeactive metal salts such as NaClO₄, NaPF₆, NaAsF₆ NaBF₄, NaSO₃CF₃,NaN(CF₃SO₂)₂ or NaN(SO₂C₂F₅)₂, One example of a suitable separator layeris 1 M LiPF₆ dissolved in propylene carbonate and impregnated in aCelgard microporous polymer membrane.

The second layer (material component) of the protective composite may becomposed of a material that is substantially impervious, ionicallyconductive and chemically compatible with the first material orprecursor, including glassy or amorphous metal ion conductors, such as aphosphorus-based glass, oxide-based glass, phosphorus-oxynitride-basedglass, sulpher-based glass, oxide/sulfide based glass, selenide basedglass, gallium based glass, germanium-based glass, Nasiglass; ceramicactive metal ion conductors, such as lithium beta-alumina, sodiumbeta-alumina, Li superionic conductor (LISICON), Na superionic conductor(NASICON), and the like; or glass-ceramic active metal ion conductors.Specific examples include LiPON, Li₃PO₄.Li₂S.SiS₂, Li₂S.GeS₂.Ga₂S₃,Li₂O.11Al₂O₃, Na₂O.11Al₂O₃, (Na,Li)_(1+x)Ti_(2−x)Al_(x)(PO₄)₃(0.1≦x≦0.9) and crystallographically related structures,Li_(1+x)Hf_(2−x)Al_(x)(PO₄)₃ (0.1≦x≦0.9), Na₃Zr₂Si₂PO₁₂, Li₃Zr₂Si₂PO₁₂,Na₅ZrP₃O₁₂, Na₅TiP₃O₁₂, Na₃Fe₂P₃O₁₂, Na₄NbP₃O₁₂, Na-Silicates,Li_(0.3)La_(0.5)TiO₃, Na₅MSi₄O₁₂ (M: rare earth such as Nd, Gd, Dy)Li₅ZrP₃O₁₂, Li₅TiP₃O₁₂, Li₃Fe₂P₃O₁₂ and Li₄NbP₃O₁₂, and combinationsthereof, optionally sintered or melted. Suitable ceramic ion activemetal ion conductors are described, for example, in U.S. Pat. No.4,985,317 to Adachi et al., incorporated by reference herein in itsentirety and for all purposes.

A particularly suitable glass-ceramic material for the second layer ofthe protective composite is a lithium ion conductive glass-ceramichaving the following composition:

Composition mol % P₂O₅ 26-55% SiO₂ 0-15% GeO₂ + TiO₂ 25-50% in whichGeO₂ 0-50% TiO₂ 0-50% ZrO₂ 0-10% M₂O₃ 0 < 10% Al₂O₃ 0-15% Ga₂O₃ 0-15%Li₂O 3-25%

and containing a predominant crystalline phase composed ofLi_(1+x)(M,Al,Ga)_(x)(Ge_(1−y)Ti_(y))_(2−x)(PO₄)₃ where X≦0.8 and0≦Y≦1.0, and where M is an element selected from the group consisting ofNd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb and/or andLi_(1+x+y)Q_(x)Ti_(2−x)Si_(y)P_(3−y)O₁₂ where 0<X≦0.4 and 0<Y≦0.6, andwhere Q is Al or Ga. The glass-ceramics are obtained by melting rawmaterials to a melt, casting the melt to a glass and subjecting theglass to a heat treatment. Such materials are available from OHARACorporation, Japan and are further described in U.S. Pat. Nos.5,702,995, 6,030,909, 6,315,881 and 6,485,622, incorporated herein byreference.

Another particularly suitable material for the second layer of theprotective composite are lithium ion conducting oxides having a garnetlike structures. These include Li₆BaLa₂Ta₂O₁₂; Li₇La₃Zr₂O₁₂,Li₅La₃Nb₂O₁₂, Li₅La₃M₂O₁₂ (M=Nb, Ta)Li_(7+x)A_(x)La_(3−x)Zr₂O₁₂ where Amay be Zn. These materials and methods for making them are described inU.S. Patent Application Pub. No.: 2007/0148533 (application Ser. No.10/591,714) and is hereby incorporated by reference in its entirety andsuitable garnet like structures, are described in International PatentApplication Pub. No.: WO/2009/003695 which is hereby incorporated byreference for all that it contains.

The composite should have an inherently high ionic conductivity. Ingeneral, the ionic conductivity of the composite is at least 10⁻⁷ S/cm,generally at least about 10⁻⁶ to 10⁻⁵ S/cm, and may be as high as 10⁻⁴to 10⁻³ S/cm or higher. The thickness of the first precursor materiallayer should be enough to prevent contact between the second materiallayer and adjacent materials or layers, in particular, the active metalof the anode. For example, the first material layer for the solid statemembranes can have a thickness of about 0.1 to 5 microns; 0.2 to 1micron; or about 0.25 micron. Suitable thickness for the anolyteinterlayer of the fourth embodiment range from 5 microns to 50 microns,for example a typical thickness of Celgard is 25 microns.

The thickness of the second material layer is preferably about 0.1 to1000 microns, or, where the ionic conductivity of the second materiallayer is about 10⁻⁷ S/cm, about 0.25 to 1 micron, or, where the ionicconductivity of the second material layer is between about 10⁻⁴ about10⁻³ S/cm, about 10 to 1000 microns, preferably between 1 and 500microns, and more preferably between 10 and 100 microns, for exampleabout 20 microns.

Seals and methods of making seals which are particularly suitable forsealing protected anodes described hereinabove and elsewhere, includingcompliant and rigid seals, are fully described in US Patent ApplicationNo.: 2007/0037058 and US Patent Application No.: US 2007/0051620 toVisco et al., and are hereby incorporated by reference in theirentirety.

Cathode Compartment

Referring again to FIG. 1, the cathode compartment 4 comprises an aircathode 5 (also sometimes referred to herein as “oxygen electrode”) andan aqueous catholyte 6, which is disposed between the cathode 5 and thesolid electrolyte protective membrane 2 and is in direct contact withthe cathode 5 for reducing molecular oxygen. The cathode compartment 4can further comprise one or more porous solid reservoir structures 7disposed between the solid electrolyte protective membrane 2 and the aircathode 5. The aqueous catholyte 6 and porous solid reservoir 7 arerepresented as separate layers in FIG. 1 for ease of illustration,however they may be and often are co-extensive in many embodiments ofthe invention.

As described further below, in various embodiments, the aqueouscatholyte (which is defined as electrolyte in contact with the cathode(e.g., oxygen electrode)) contains dissolved and, in at least somecases, active catholyte salts.

Aqueous Catholytes

In various embodiments, aqueous catholytes of the present inventioncomprise the following components:

1) a dissolved active catholyte salt, which participates in the cathodedischarge;

2) a dissolved supporting Li salt, which does not directly participatein the cathode discharge;

3) optionally, undissolved catholyte salt(s);

4) optionally, an additional hygroscopic agent;

5) optionally, additives enhancing cell performance.

One class of active catholyte salts of the current invention has aformula MHal_(n), where M is NH₄, Al, Ti or Mg; Hal is Cl, Br or I; andn has the appropriate stiochiometric value based on the valence of M.The preferred salts are NH₄Cl and AlCl₃. The term “salt” as used in thiscontext is intended to encompass compounds that react (e.g., dissolve orhydrolyze) to form dissolved active salts. Other active catholyte saltsthat can be effectively used in Li air cells are NH₄NO₃ and NH₄BrNH₄CNS. All of these salts participate in the cathode process formingtheir corresponding Li salts. During cathode discharge, theircorresponding hygroscopic Li salts form and prevent the drying out ofthe cell by absorbing moisture from air.

An exemplary active salt of the instant invention is NH₄Cl. Dissolved inthe catholyte, the active salt participates in the cell reaction. Thedischarge reaction with ammonium chloride, in dilute conditions, mayappropriately be described as follows:2Li+2NH₄Cl+½O₂═H₂O+2NH₃+2LiCl

In accordance with the above reaction, water and ammonia (and/orammonium hydroxide) is generated as the active salt, partaking in thedischarge reaction, is converted to a corresponding lithium saltdischarge product (LiCl). Preferably, the corresponding lithium salt ishighly soluble in the catholyte, and preferably more soluble than thatof the active salt. One metric for gauging the solubility of the activesalt or that of its corresponding lithium salt is based on watersolubility. Accordingly, in various embodiments, the solubility in waterof the corresponding lithium salt is greater than the water solubilityof the active salt from which it was converted. In various embodimentsit is preferable to use an active salt that converts, during discharge,to a corresponding hygroscopic lithium salt that imparts to thecatholyte a low equilibrium relative humidity. For instance theequilibrium relative humidity of a saturated water solution of LiCl isabout 11% and LiBr is about 6% at room temperature (20° C.).

High concentrations of active catholyte salts, much higher than aretypically used in conventional metal/air (e.g., Zn/air) cells aredesirable in order to increase the amount of water collected by thehygroscopic solid phase discharge products. Concentrations of at least1M, or at least 2M, for example 4M, are suitable. In specificembodiments, a statured or near saturated solution of the active salt inthe catholyte can be used.

Particular Salts

NH₄Cl, NH₄Br and NH₄I

Solubility of NH₄Cl in water at room temperature (RT) is approximately28 wt % and increases with temperature, which is significantly largerthan solubility of LiOH (approx. 11 wt % at RT). At the same time, bothdischarge products, LiCl and NH₃, have very high solubilities. LiCl ishygroscopic and absorbs moisture from atmosphere during cell storage anddischarge. Importantly, during cell discharge each mole of NH₄Cl alsogenerates half a mole of H₂O, which can be used during further dischargeinvolving formation of LiOH. NH₃ can evaporate, thereby decreasing theweight of the cathode compartment. Use of NH₄Br or NH₄I leads toformation of LiBr and LiI, which are much more hygroscopic than LiCl,therefore these salts can be used and may be particularly useful when acell has to operate at low humidity.

An additional benefit of the NH₄Cl salt is associated with buffering thecatholyte during cell storage and initial discharge. Since solidelectrolyte protective membranes are chemically stable in NH₄Cl-basedelectrolytes, the use of NH₄Cl as the active salt results in improvedstorage performance of the battery cell.

NH₄NO₃

NH₄NO₃ has a very high solubility of 68 wt % at RT and the dischargeproduct, LiNO₃, is very hygroscopic and even more soluble than LiCl.Additionally, air cathodes in cells with NH₄NO₃ active salt can havetraditional inexpensive Ni current collectors, since Ni is stable inLiNO₃-containing solutions.

AlCl₃

AlCl₃ has a high solubility of 31 wt %; the discharge products arehygroscopic and prevent the drying out of the cell. It has been foundthat cells with this salt have an increased depth of discharge.

Supporting Li Salts

Supporting Li salts of this invention maintain conductivity of catholyteat different stages of discharge. It has also been found thatintroduction of certain concentrations of Li salts, substantially higherthan conventional Li salt concentrations, into the catholyte prior toinitial discharge prevents a rise in the cell's resistance duringstorage under open circuit conditions. In this regard, initial (prior toinitial discharge) Li salt (ion) concentrations of at least 2M, forexample 2M, can be effectively used. While the invention is not limitedby any particular theory of operation, this effect is attributed to thesuppression of ion exchange at the interface between protective membraneand liquid catholyte. Hygroscopic supporting Li salts can be used inorder to maintain moisture balance in the cell before the start ofdischarge. This is one more function of supporting Li salts.

In a specific case, the catholyte of the current invention comprises theactive salt and the supporting salt with the same anion (common anion),i.e. NH₄Cl or/and AlCl₃ are used in combination with LiCl or incombination with LiBr (e.g., NH₄Cl with LiBr), NH₄NO₃ with LiNO₃, andNH₄CNS with LiCNS. Depending on the particular application and theconditions under which the battery will operate, the catholyte of thecurrent invention may comprise the active salt only and not comprise anyconcentrations of the supporting salt or may comprise the supportingsalt only and not comprise any concentrations of the active salt. Forinstance, in applications where the evolution of gaseous NH₃ during celldischarge is not desirable, the catholyte can contain LiCl or/and LiNO₃,but not NH₄Cl or NH₄NO₃. Some specific compositions of the catholytes ofthe current invention are: 4M NH₄Cl+2M LiCl in water, 4M NH₄NO₃+2M LiNO₃in water, 4M NH₄CNS+2M LiCNS in water, 2M AlCl₃+2M LiCl in water, 2.7MAlCl₃+1M LiCl in water.

In some embodiments, active salts and supporting Li salts with differentanions are used. In one embodiment, hygroscopic LiBr or LiI are used asLi supporting salts in combination with NH₄Cl and NH₄NO₃ active salts inorder to prevent the drying out of the cell when discharging at lowhumidities. In some embodiments, the catholyte contains additionalhygroscopic agents or co-solvents other than the Li salts. Oneparticular example of such co-solvent is ethylene glycol, which canabsorb as much water as 200% of its weight at 100% relative humidity.

In some cases, fine particles of inert solid compounds, such as silicaor alumina, are added to the catholyte or to the cathode compartment indifferent locations: near the cathode surface, in the bulk of thecatholyte reservoir, near the protective membrane surface. Theseparticles create crystallization centers and control the precipitationof the discharge product.

In other embodiments the catholyte further comprises an inorganic liquid(other than water), including aprotic and protic organic and inorganicliquids (TiCl₄) that reacting with water of the catholyte form dissolvedactive species that participate, as a reagent, in the cell reaction.

In various embodiments the catholyte has a low equilibrium relativehumidity (ERH) prior to initial discharge. Preferably the ERH of thecatholyte, prior to initial discharge, is less than 50%, less than 40%,less than 30%, less than 20% or less than 15%, at room temperature (20C). And preferably, it is low enough to render the cathode compartmenthygroscopic upon cell activation.

Low ERH values may be imparted to the catholyte using highconcentrations of active salts or saturating the catholyte with activesalts, or using a combination of dissolved active salts and non-activesalts, particularly non-active lithium salts, and especially lithiumhalides, which are extremely hygroscopic. In various embodiments, thecatholyte comprises a combination of a highly concentrated active salt(e.g., NH4Cl) and a lower concentration of a lithium halide salt (e.g.,LiCl, LiBr or LiI). Salt combinations can be used to effectively lowerthe solubility limit of one or both salts. For instance, a highconcentration of an active first salt can be used to lower thesolubility of a second salt (e.g., lithium salt), and by this expedientthe lithium salt may reach its solubility limit in the catholyte at aconcentration below its solubility limit in pure water. In effectiveconcentrations, the combination of a first and second salt, or moresalts, interacting in their dissolved form can lead to early saturationat concentrations lower than that which would be expected based solelyon their solubility limits in water.

In various embodiments, aqueous catholyte, including that whichcomprises an active salt dissolved therein, is incorporated in thecathode compartment during battery cell manufacture and in a sufficientamount to fill the pores of the reservoir layer, wet the cathode activelayer, wet the surface of the protective membrane and imbibe a hydrogel,when present.

In certain embodiments, catholyte may be introduced into the compartmentafter cell manufacture but prior to initial cell discharge. By thisexpedient, the battery cell may be assembled, stored and transported ina “dry state” (i.e., substantially without, or free of, or in theabsence of, catholyte and water) or in a semi-dry state with less, orsubstantially less, catholyte (or water) than that which is optimal forpore filling, wetting and the like. The advantages of assembling,storing and transporting a dry state cell include exceptionally lighttransport, optimal transport safety, and enhanced shelf life. In certainembodiments, it is contemplated that an operator (typically a personwhom is using the battery to power a system or device, but not limitedas such, and mechanical apparatus' are contemplated herein for that samepurpose) introduces catholyte or water via an entry port in the batterycell case. The port provides external access to the cathode compartmentfor various intended uses, including the introduction of water, oractive catholyte, or salts. For instance, in certain embodiments, thecathode compartment contains at least one of an active solid phase saltor a solid lithium salt, or both, and a battery operator introduceswater and/or catholyte into the cathode compartment via the entry port.

In certain embodiments, the catholyte is wholly formed, or partlyformed, in situ upon exposure of the cathode compartment to ambient air.For instance, the cathode compartment comprising at least one solidphase salt, for instance an active solid phase salt and/or a solid phasesupporting electrolyte salt (e.g., LiCl) that, subsequent to cellactivation, dissolves in contact with water absorbed from the ambientair. In certain embodiments thereof, the cathode compartment issubstantially devoid of water prior to cell activation. In specificembodiments all the water necessary for cell discharge is obtainedthrough absorption from the ambient air.

Advantages of Catholytes of the Current Invention

The Li/air cells of the present invention are in some ways analogous tothe classic Zn/air cell with the alkaline KOH aqueous electrolyte. Byanalogy, the most natural choice of a catholyte for the Li/air cellwould be aqueous LiOH. The neutral or slightly acidic catholyte of thecurrent invention has several advantages over LiOH-based electrolytes.

First, solubility of LiOH in water at RT is only 11 wt %, far less thanfor aqueous KOH or active catholyte salts of the current invention. Theair cell with such catholyte would deliver an insignificant capacitybefore LiOH starts to precipitate out. The use of a catholyte inaccordance with the current invention allows for an increase in thecapacity delivered prior to formation of solid discharge product.

Second, the discharge products of a cell with the catholytes of thecurrent invention, such as LiCl, LiNO₃ and LiCNS, are much morehygroscopic than LiOH and therefore prevent cell drying much better.Importantly, solid electrolyte protective membranes are much more stablein contact with catholytes of current invention than in contact withLiOH, resulting in increased storage stability of the cell.

Also, it has been found that when a cell with the catholyte describedherein is deeply discharged and precipitation of LiOH takes place, thetotal delivered capacity is significantly larger than could be expectedfrom adding the capacity of active salt discharge (i.e., when NH₄Cltransforms into LiCl) to the capacity of LiOH discharge. While theinvention is not limited by any particular theory, this synergisticeffect can be explained by an increase in conductivity and animprovement of morphology of the solid discharge product formed whenLiOH and LiCl co-precipitate. Another explanation of this effectinvolves the high hygroscopicity of the discharge products, which absorbwater and dilute LiOH as well as Li₂CO₃ formed when LiOH reacts withatmospheric CO₂.

Introduction of Solid Active Catholyte Salts into the CathodeCompartment

In another specific embodiment of the current invention active salts areintroduced into the cathode compartment as solid phase active salt. Whenpresent, at least a portion of the solid phase active salt contacts thecatholyte, and operably dissolves therein. In certain embodiments, theactive solid phase salt is introduced into the cathode compartment inconcentrations larger than its solubility limit in the catholyte And bythis expedient, water necessary for the cell discharge need not beinitially loaded into the cell, but may rather be absorbed into thecathode compartment from the atmosphere by the hygroscopic productsformed during discharge. An important advantage of certain active solidphase salts is a decrease in battery cell weight. The active solid phasesalts also serve to keep the active salt concentration in the catholytehigh during cell operation

In various embodiments some amount (at least a portion) of the solidphase active salt remains solid and in contact with the catholytesubsequent to initial discharge, and typically a portion remains solidand un-dissolved for a significant fraction of the discharge thereafter.In certain embodiments, subsequent to the cell delivering at least 10%,or at least 30%, or at least 50%, or at least 75%, of its ratedcapacity, or substantially 100% of its rated capacity, there exists inthe cathode compartment a portion, or at least some amount, of activesolid phase salt, still un-dissolved and in contact with catholyte.

During cell operation, when active solid phase salt is present in thecathode compartment, the catholyte may reach a very low equilibriumrelative humidity, and, driven by the continual formation of hygroscopicsolid discharge products, the catholyte may maintain those low valuesfor a significant fraction of the total discharge.

In various embodiments, the catholyte reaches a low equilibrium relativehumidity of less than 50%, or less 40%, or less than 30% or less than20% or less than 15%, and in certain embodiments the catholyte maintainsthat ERH value over a significant fraction of the discharge, includingthat period which corresponds to the cell delivering over 50% of itsrated cell capacity, or over 60%, or over 70%, or over 80%, or over 90%,or over the entire discharge.

In various embodiments, the equilibrium relative humidity of thecatholyte remains less than or substantially equal to the ambientrelative humidity (RH) for a significant fraction of the total timeperiod (TTP) in which the cell operates. The total time period refers tothe sum total of time between the start of initial discharge until thedischarge is substantially complete, and includes periods of rest whenthe cell is under open circuit conditions. Preferably, the ERH of thecatholyte is below or the same as the RH of the ambient air for morethan 50% of the TTP, more preferably 75% and even more preferably 90%.In certain embodiments the ERH remains below the RH for the entire timeperiod of cell operation (100% TTP).

Typically, the active solid phase substance (active solid phase salt) isincorporated in the cathode compartment during cell manufacture,although the invention is not limited as such and it is contemplatedherein that the active solid phase salt may be loaded at a timesubsequent manufacture, including: subsequent to manufacture and priorto cell activation; or subsequent to cell activation and prior toinitial discharge; and it is also contemplated that active solid phasesalt is loaded into the cathode compartment after initial discharge.Notably, in accordance with the instant invention, the active solidphase substance (active solid phase salt) is intentionally incorporatedin the cathode compartment as a solid (i.e., solid state of matter), andas such it is not formed solely as the result of unintentional, or evenundesirable, phenomena, such as low temperature precipitation reaction,which is a reaction in which solid salts precipitate when the solutionin which they are dissolved is lowered, usually to a temperature belowroom temperature, or other unintentional causes. And while lowtemperature precipitation may, in fact, take place in the cells of theinstant invention when operated or stored below room temperature, themajority of the active solid phase salt present in the cathodecompartment is not formed by an unintentional mechanism, and moretypically 90% or more of the solid phase active salt is incorporated inthe cathode compartment, with the distinct intent of doing so, andusually during cell manufacture or prior to initial cell activation.

In various embodiments the active solid phase substance (typically anactive solid phase salt) is present in the cathode compartment inrelatively large amount prior to initial discharge, or, in some cases,prior to cell activation. In various embodiments prior to cellactivation or initial discharge the solid phase active salt is presentin the cathode compartment in a larger amount than active salt dissolvedin the catholyte.

In various embodiments, the mole ratio of the solid phase active salt tothe active salt dissolved in the catholyte is greater than 1, or greaterthan 2, or even greater than 10.

In specific embodiments, the mole ratio has a value in the range of:from 2 to 3; from 3 to 4; from 4 to 5; from 5 to 6; from 6 to 7; from 7to 8; from 8 to 9; or from 9 to 10.

Methods of Loading the Cathode Compartment with NH₄Cl and Other SolidActive Salts of the Current Invention

1) Filling the pores of the reservoir layer by air spraying a slurry orsolution of NH₄Cl or another active salt. In current invention, thesolution or slurry are based on water or its mixture with one or moreco-solvents. Such co-solvents, both protic and aprotic, can be used toenhance the solubility of the active salt, improve the wettability ofthe reservoir layer, or disperse the solid salt particles. In onepractically significant embodiment, methanol is added to the slurry towet the porous reservoir layer during impregnation. One or moreimpregnation/drying cycles are used to fill the pores of the reservoirlayer. In one particular embodiment, the reservoir layer is heatedduring impregnation.2) Vacuum impregnation of the reservoir layer with NH₄Cl or anotheractive salt using its solutions or slurries in aqueous and non-aqueoussolvents and their mixtures.3) Impregnation of the reservoir layer with NH₄Cl or another active saltby placing the reservoir layer in a bath with hot slurry or solution,followed by cooling and crystallization. In one embodiment, thereservoir filled with slurry or solution is cooled down to thecryohydrate point. As a result, the solution separates into two solidphases: NH₄Cl and ice. Then, water is extracted with a solvent havingnegligible solubility of NH₄Cl (acetone).4) Coating the air cathode surface with slurry of NH₄Cl or anotheractive salt.5) Pressing NH₄Cl or another active salt into pellets or thin layers andplacing them into the cathode compartment in contact with the aircathode. In one embodiment, the active salt is mixed with binders priorto the pressing operation. In another practically important embodiment,the active salt is mixed with inert inorganic powders, such as silica,alumina, etc. prior to the pressing operation in order to increase thearea of grain boundaries of the polycrystalline active salt and toenhance its ionic conductivity. Other types of fillers, such as shortcarbon fibers, also can be used. In another embodiment, the active saltis mixed with powders of pore-forming agents prior to the pressingoperation. After pressing, the pore-forming agents are removed viathermal decomposition or selective solubilization with non-aqueoussolvents. As a result, porous active salt pellets or layers are formed.In one particular embodiment, ammonium bicarbonate with a lowdecomposition temperature of about 60° C. is used as a pore-formingagent.

In various embodiments, when the solid active salt is introduced intothe cathode compartment, aqueous solution of the Li supporting salt orboth Li supporting salt and active salt is also loaded into the cathodecompartment. It can be achieved by filling the pellets of the solidactive salt or the first reservoir layer (already impregnated with solidactive salt) with the aqueous solution of the Li supporting salt or bothLi supporting salt and active salt. Alternatively or additionally, itcan be achieved by filling the second reservoir layer located betweenthe protective membrane and the first reservoir layer with the sameaqueous solution. As a result, the components of the cathode compartmentand the protective membrane are in contact via liquid phase.

In an important embodiment, the amount of active salt introduced intothe cathode compartment as a solid is significantly larger than theamount of active salt in the dissolved form, prior to initial discharge.

Discharge Stages

In various embodiments the discharge may be described as taking place indifferent stages, based on whether or not dissolved active salt speciesparticipate, as a reagent, in the discharge reaction: a first stagecorresponding to that portion of the discharge when it does and thesecond stage to that portion when it does not; for instance, the secondstage is reached once substantially all of the dissolved active saltspecies have been utilized, or those species may not be present in thecatholyte. Although not limited as such, typically water is generated bythe cell reaction during the first stage of discharge and water isconsumed by the reaction during the second stage.

In various embodiments lithium/air cells of the instant inventionencompass both first and second stages of discharge. Accordingly, invarious embodiments, the rated capacity of the battery cell comprisesthe capacity delivered over the first and second stages of discharge.And in certain embodiments the rated capacity of the cell is equivalentto the sum total of the capacity delivered over the first and secondstages. The invention, however, is not intended to be limited as such,and in some embodiments the cell may be exclusively discharged in one orthe other of the first or second stages. Moreover, it is to beunderstood that the use of the term first and second is not intended toimply that the second stage necessarily follows the first stage, albeitthis is usually the case, or that the first stage of discharge isnecessary for second stage discharge.

Water molecules generated during the first stage of discharge may,without limitation, coalesce with the existing water in the catholyte orthey may compound with constituents of the catholyte to form astructural hydrate (e.g., a hydrated solid salt, the water thereinstructural). The total amount of water reactively generated during firststage discharge is proportional to the sum total of active catholytespecies that participate, as a reactant, in the discharge.

In accordance with embodiments of the instant invention the cathodecompartment comprising solid phase active salts and/or dissolved activesalts may be formulated with the objective of preventing catholyte dryout and improving cell performance, including enhancing specific energyand energy density. As described above, by making use of highlyconcentrated or saturated catholyte salt solutions, or certain saltcombinations in the catholyte, or solid phase active salts, or somecombination thereof, the cathode compartment may be rendered hygroscopicand sufficient amounts, and even large amounts, of water moisture, fromthe ambient air, absorbed over various periods of discharge. By thisexpedient, the energy density (Wh/kg and Wh/l) of the cell may beenhanced, in part, because less than the requisite amount of waternecessary for the cell to achieve its rated discharge capacity is neededin the cathode compartment prior to initial discharge; or in certainembodiments prior to cell activation.

In accordance with various embodiments of the instant invention at leastthree different amounts of water molecules can be distinguished based onhow or when that amount of water materializes in the cathodecompartment: a first amount of water corresponding to that which ispresent in the catholyte prior to initial discharge; a second amount ofwater corresponding to that which is generated by the dischargereaction; and a third amount of water corresponding to that which,subsequent to the start of initial discharge, is absorbed by thecatholyte from the ambient air. The total amount of water consumed bythe discharge reaction is considered herein as corresponding to a fourthamount. And a fifth amount of water corresponds to that amount of waterwhich is absorbed from the ambient subsequent to cell activation andprior to initial discharge.

In various embodiments, the first amount of water, or the sum of thefirst and second amount of water, is insufficient for the cell toachieve its rated discharge capacity. And for those embodiments, it isonly through the absorption of a sufficient amount of water moisture,i.e., the third amount of water, that the cell is able to deliver itsrated capacity on discharge. By manufacturing a cell with less waterthan that which is needed for full (complete) discharge, the cells ofthe instant invention are made lighter and more volumetrically efficientthan would otherwise be possible. In various embodiments, the thirdamount of water is greater than the first amount of water, or greaterthan the sum of the first and second amount of water. In certainembodiments, the third amount of water is at least twice that of thefirst amount of water, or at least twice that of the sum of the firstand second amount of water. In certain embodiments the cell is devoid ofwater prior to cell activation, and a sufficient amount of water isabsorbed from the ambient prior to initial discharge to enable the cellto begin operation, (i.e., the fifth amount of water thereinsufficient).

Solid Porous Structures as Reservoir Layers

It has been found that several types of porous structures can beeffectively used as reservoir layers in Li/air aqueous cells inaccordance with the present invention. These porous layers arechemically inert and compatible with the cathode and with the aqueouscatholyte. In particular, they do not react with the catholytecomponents, cannot be oxidized by the cathode, and do not participate inthe cell discharge as reagents. The first function of a porous reservoirdisposed between the cathode and the protective membrane is to be loadedwith liquid catholytes and/or solid phase salts. Solid structure(s) usedas reservoir layer(s) have high porosity. It has also been found thatreservoir layers can have a second function of accommodating both liquidand solid cell discharge products, thereby increasing the depth ofdischarge and improving cell characteristics. Additionally, the porousspace of the reservoir layer retains water that is absorbed by thehygroscopic components of the catholyte during cell discharge andstorage thereby making it available for discharge reactions.

Metal Oxide Porous Reservoirs

A species of metal oxide porous reservoir structure has previously beendisclosed in applicants' prior published US Application US 2004/0197641.In this application, porous ZrO₂ (in particular, Zirconia cloth fromZircar Products, Inc.) was noted in this context. It has now beendetermined that several specific ZrO₂ porous reservoir structures aresuitable in accordance with the present invention, including a highporosity (>95%) zirconia felts ZYF-150, ZYF-100 and ZYF-50 from ZircarZirconia Corp. In addition, metal oxides including Al₂O₃, Y₂O₃, MgO,etc. as felts, cloths and other porous structures are suitable. Inparticular, high porosity (>95%) alumina felts ALF-100 and ALF-50 fromFuel Cells Materials Corp. can be used.

A feature of porous reservoirs in accordance with the present inventionis their ability to accommodate solid phase discharge productprecipitation without adverse impact on cell function, or at least sothat cell function is retained at an acceptable level despite solidphase discharge product precipitation. In order to accomplish this, thethickness of porous reservoir structures is much greater than thattypically used in the context of Zn/air batteries, the closestcommercially available corollary and typical starting point for othermetal/air battery designs.

Carbonaceous Porous Reservoirs

Carbon and graphite cloths and felts, carbon papers and other porousstructures are suitable as solid porous reservoir structures inaccordance with the present invention. WDF graphite felt and VDG carbonfelt from National Electric Carbon Products, Inc. and carbon felt fromFiber Materials, Inc. can be used. Carbonaceous materials can befabricated to have a narrow tailored porosity or a graded porosity tooptimize catholyte penetration and/or accommodation of solid phasedischarge product precipitation without adverse impact on cell function,or at least so that cell function is retained at an acceptable leveldespite solid phase discharge product precipitation.

Not previously used in this context, carbonaceous reservoir structureshave characteristics that confer additional benefits in Li/air cellimplementations. Carbonaceous structures are lightweight, therebyimproving cell energy density. They are also electronically conductive.The electronic conductivity of these reservoir structures can be used tohelp control the amount and location of solid phase cell dischargeproduct precipitation.

Several methods can be used to enhance wettability of carbonaceousporous reservoirs, especially when they are used in combination withneutral or basic catholytes: treatment with hot acid (HCl or H₂SO₄),electrochemical pre-cycling of the carbonacious reservoirs or heattreatment in an oxygen-containing atmosphere. A specific method oftreatment of carbon felts is heat treatment at 525-565° C. in air for2-2.5 hrs.

Polymeric Porous Reservoirs

Polymeric layers with high porosity (e.g., at least 50%, for example90%) can be used as reservoir structures. An example of such a reservoiris polypropylene fiber materials. Another important example ispolyurethane foam, particularly reticulated foam. In a preferredembodiment, these materials are elastic and can expand during dischargewith minimal damage to their porous structure and can retain liquid andsolid discharge products, as well as water absorbed from air during celldischarge while cell function is retained at an acceptable level. Inanother preferred case, the polymeric reservoir structures are elasticenough to expand during discharge and keep all the cell components incontact with each other, obviating the need for separate mechanisms(such as springs) for accomplishing this in the assembled battery cell.These elastic polymeric porous reservoir structures can be particularlysuitably combined with a compliant seal cell structure such as disclosedin applicants' copending published Application No. US 2007/0037058,incorporated by reference herein for this purpose.

Optionally, a second porous reservoir layer can be used in the cathodecompartment in order to improve the contact between the solidelectrolyte protective membrane and the first reservoir layer in thecase when it is impregnated with solids. In this case, the second porouslayer is also filled with liquid catholyte. Another function of thesecond reservoir layer can be to prevent or minimize the precipitationof the solid discharge products at the protective membrane. In thiscase, the porous structures of the first and second reservoir layers areconfigured such that solid phase discharge products will preferentiallyprecipitate in the second reservoir layer (i.e., away from thereservoir/cathode interface). In another embodiment, the second porousreservoir layer optimizes the structure and conductivity of theprecipitate near the surface of the protective layer. Accordingly, thefirst porous reservoir layer is made from a first component material andhas a first pore structure, and the second porous reservoir layer ismade from a second component material and has a second pore structure.In various embodiments, to establish preferential precipitation in oneor the other of the first or second reservoir, the first pore structureis different from that of the second pore structure, and the firstcomponent material may also be different than the second componentmaterial. The pore structure may have uniform or varying pore size andpore shape. The porous solid components of the cathode compartmentgenerally have an open porosity of at least 30%, more preferably atleast 50%, even more preferably at least 70% and yet even morepreferably at least 90%.

Hydrogels as Reservoir Layers

It has been found that hydrogels, hydrophilic polymer networks that canabsorb water, can be effectively used in Li/air aqueous cells asreservoir layers in accordance with the present invention. Hydrogellayers of the current invention are compatible with the cathode and withthe aqueous catholyte. In particular, they do not react with thecatholyte components, cannot be oxidized by the cathode, and do notparticipate in the cell discharge as reagents. The hydrogel reservoirlayer disposed between the cathode and the solid electrolyte protectivemembrane can be loaded with active and supporting catholyte salts in thedissolved form and possibly supplemented by and catholyte salts in theform of undissolved solids. The hydrogel reservoir layers of the currentinvention serve several other purposes. In particular, they canaccommodate both liquid and solid products of cell discharge, therebyincreasing the depth of discharge. Additionally, the hydrogel reservoirlayers can retain the water that is absorbed by the hygroscopiccomponents of the catholyte or hygroscopic products of the celldischarge. This is particularly beneficial when the hydrogel reservoirlayers are loaded with solid active catholyte salts and the dischargeproducts are highly hygroscopic. In this case, the hydrogel reservoirlayers absorb water and swell, thereby preventing leakage from thecathode compartment.

The composition and synthesis of hydrogels are described in manypublications. An extensive list of materials suitable for use in thisinvention can be found in I. R. Scott and W. J. Roff, Handbook of CommonPolymers, CRC Press, 1971, incorporated by reference herein for thispurpose. The hydrogels of the current invention can be based on bothnatural and synthetic polymers. They can be physical gels or chemicalgels. The hydrophilic polymers used to synthesize hydrogel matrix andthe methods for synthesizing physical and chemical hydrogels are listedin the following publications: B. D. Ratner and A. S. Hoffman, In:Hydrogels for Medical and Related Applications, American ChemicalSociety, Washington, D.C., 1976, pp. 1-36; A. S. Hoffman, Advanced DrugDelivery Reviews Vol. 43, pp. 3-12 (2002), which are incorporated hereinby reference for this teaching. Suitable hydrophilic polymers include,but are not limited to polyesters, poly(vinyl alcohol) andpolyacrylamide. In one particular embodiment the hydrogel reservoirlayer based on crosslinked polyacrylamide is fabricated by casting thegel electrolyte prepared by adding ammonium persulfate (as apolymerization initiator) and N,N,N′,N′_Tetrmethylenediamine (as anaccelerator) to the mixture of acrylamide monomer and bis-acrylamidecrosslinker dissolved in water. In one case, an active salt and asupporting Li salt are also added to that solution in suchconcentrations that the formed hydrogel electrolyte layer containscompletely dissolved active and supporting salts. In another case, priorto crosslinking the solution is loaded with solid active salt in aconcentration greater than the salt's solubility limit, so that theresulting hydrogel layer contains undissolved solid active salt anddissolved supporting salt. In one embodiment, the concentration of thesolid active salt is not constant, but rather is changing across thereservoir hydrogel layer. In another embodiment the salt concentrationis higher near the cathode and lower near the protective membrane.

In the current invention, multi-layer hydrogels can be used to improvethe battery cell's characteristics. In this case, the reservoir layerscomprise two or more hydrogels based on different or identical polymersand containing different or identical active and supporting catholytesalts. In one embodiment, in order to improve the contact between thecathode and the hydrogel layer loaded with solid active salt particles,this layer is coated with another thin hydrogel layer containing nosolid active salt, which is in direct contact with the cathode. Inanother embodiment, the thin hydrogel layers containing no solid activesalt are disposed not only between the hydrogel layer loaded with thesolid active salt and the cathode surface, but between the hydrogellayer loaded with the solid active salt and the solid electrolyteprotective membrane as well.

Air Cathodes

In embodiments of the invention, an air cathode analogous to that usedin Zn/Air batteries or low temperature fuel cells (e.g., PEM), and whichare well known to those of skill in that art, may be used as the aircathode in the inventive Li/air battery cells described herein.

In various embodiments, the instant Li/air cell comprises an inventiveair cathode comprising, at least, a first sectional layer comprising afirst gas diffusion (e.g., Teflon) backing layer (which is positionedadjacent to the air side in the cell), a wet-proof gas-supply layer, forexample made of Teflon and acetylene black, a metal screen currentcollector and an active carbon layer.

The type of metal used for the current collector may be chosen based onits chemical stability in the cell, specifically its stability incontact with the aqueous catholyte. In specific embodiments the metalscreen current collector is titanium.

The active carbon layer may contain an electrocatalyst or may beuncatalyzed. The following catalysts can be used in the air cathodes ofthe current invention: oxides of Mn, Co and Ru and other metal oxides;cobalt phthalocyanines, iron phthalocyanines and manganesephthalocyanines.

In certain embodiments the inventive air cathode comprises the firstsectional layer and a second sectional layer, typically a porouscarbonaceous structure, adjacent to it. In specific embodiments theactive layer provides the first section surface forming the interfacewith the second section. By this expedient the active layer is disposedin the bulk of the air cathode.

Porous carbon structures particularly suitable for use as a secondsectional layer include graphite cloths and felts and carbon papers, andthe like.

In various embodiments the first and second sectional layers are adheredor bonded at the interface, thereby forming what is termed herein as aunitary cathode structure. Alternatively, the two sections may be placedside by side in the cell, whereupon they adhered to each other duringdischarge, and by this expedient form, in-situ, the unitary structure.As described hereinabove, the first and second sectional layers may forma unitary cathode structure in contact, however the invention is notlimited as such, and additional material layers may be disposed as acomponent material layer in the unitary structure, between the first andsecond layers.

The inventive cathode serves to accommodate solid products that form inthe cell during discharge. Another interesting feature of the inventiveair cathode is that the active carbon layer, where the cathodeelectroreduction takes place, is disposed in the bulk of the cathode andthe cathode expands upon discharge. By this expedient, the inventivecathodes of the instant invention are particularly suitable for highcapacity Li/Air cells because the cathode, expanding on discharge, forexample a 2-fold increase in thickness, or as much as a 10 fold increasein thickness or more, will continue to accommodate solid product as itforms.

In one embodiment, when the active catholyte salt is of the typeMHal_(n) (in particular, NH₄Cl and AlCl₃), the Ti current collector isused instead of the traditional Ni current collector in order to avoidcorrosion.

In Li/air batteries, the discharge products have to be accommodatedwithin the cathode compartment. Air cathodes of the same structure asthe air cathodes of the Zn/air batteries are unable to accommodate largeamounts of the discharge products. Cathodes with traditional structurecan be used in combination with porous reservoirs described in thecurrent invention. Alternatively, the oxygen electrode has a porestructure configured to accommodate insoluble discharge products suchthat operation of the cell is not disrupted prior to substantiallycomplete discharge.

In the case when the reservoir layer is a carbonaceous porous structureplaced in direct contact with the active layer, we effectively form anew carbon-based air cathode. The resulting air cathode has a uniquestructure. It employs an active layer in the depth of the electroderather than on the surface as in traditional air cathodes. Carbonaceousporous reservoir is uncatalyzed and has a relatively large resistance,so the electroreduction mostly occurs at the surface of the active layerin the depth of the electrode, keeping the discharge products away fromthe protective solid electrolyte membrane. The resulting air cathodefunctions as a gas diffusion electrode and a carbonaceous porousreservoir at the same time. The proposed new cathode is one of thesubjects of the current invention.

In one embodiment, the cathode is fabricated outside of the cell priorto its assembly. In another important embodiment, the cathode isfabricated by bringing the gas diffusion electrode and the carbonaceousporous reservoir in intimate contact during cell assembly.

EXAMPLES

The following examples provide details illustrating advantageousproperties of cells in accordance with the present invention. Theseexamples are provided to exemplify and more clearly illustrate aspectsof the present invention and in no way intended to be limiting.

Discharge tests were performed in the Li/Air cells employingdouble-sided solid electrolyte-protected Li anode with flexible seal andthe cathode compartment components of the current invention. The cellswere placed in the Environmental Chamber at 25° C. and relative humidityof 50%. Each cell had one double-sided anode located between two cathodecompartments having identical components. The solid electrolytemembranes had the size of 25.4 mm×25.4 mm. The cathode compartmentcomponents had the following sizes: air cathode: 26 mm×26 mm; porouscatholyte reservoir layers or hydrogel reservoir layers: 25.4 mm×25.4mm. The air cathodes employed the gas-diffusion Teflon backing layershaving a Gurley number of 2000.

Example 1

FIG. 3 illustrates performance of a Li/Air cell having 4M NH₄Cl, 2M LiClcatholyte and zirconia felt reservoir layers.

Example 2

FIG. 4 illustrates performance of a Li/Air cell having 4M NH₄Cl, 2M LiClcatholyte and graphite felt reservoir layers.

Example 3

FIG. 5 illustrates stability of a solid electrolyte protective membranein contact with 4M NH₄Cl, 2M LiCl catholyte during long-term storage.

Example 4

FIG. 6 illustrates comparative performances of Li/Air cells having 4MNH₄NO₃, 2M LiNO₃ and 1M LiOH catholytes.

Example 5

FIG. 7 illustrates performance of a Li/Air cell with 2.7M AlCl₃, 1M LiClcatholyte and alumina felt reservoir layers.

Example 6

FIG. 8 illustrates performance of a Li/Air cell with 2.7M AlCl₃, 1M LiClcatholyte and graphite felt reservoir layers.

Example 7

FIG. 9 illustrates performance of a Li/Air cell having alumina feltreservoir layers impregnated with solid NH₄Cl salt and additionallyfilled with 4M NH₄Cl, 2M LiCl catholyte.

Example 8

FIG. 10 illustrates performance of a Li/Air cell having pressed pelletsof NH₄Cl salt mixed with alumina powder and Whatman micro fiber filtersGF/A as second reservoir layers filled with 4M NH₄Cl, 2M LiCl catholyte.

Example 9

FIG. 11 illustrates performance of a Li/Air cell having crosslinkedpolyacrylamide hydrogel reservoir layers containing dissolved 4M NH₄Cland 2M LiCl.

Example 10

FIG. 12 illustrates performance of a Li/Air cell having crosslinkedpolyacrylamide hydrogel reservoir layers containing dissolved 2M LiCland loaded with solid NH₄Cl.

Alternative Embodiments

While the invention is described primarily in terms of Li and Li alloysanodes, other alkali metal anodes, in particular sodium (Na) may also beused in alternative embodiments. In such an alternative embodiment, theprotective membrane architecture on the anode is configured for highionic conductivity of the alkali metal ions of the anode material. Forexample, a protective membrane architecture for a Na metal anode mayinclude a solid electrolyte layer composed of Nasicon.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theinvention. It should be noted that there are many alternative ways ofimplementing both the devices and methods of the present invention.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein. The following sample claims recite somenon-exhaustive aspects of the invention to be claimed.

1. A battery cell, comprising: a protected anode, comprising, a lithiummetal anode having a first surface and a second surface, a protectivemembrane architecture on at least the first surface of the anode, acathode compartment comprising a cathode for reducing molecular oxygen,an aqueous catholyte, and a hydrogel reservoir structure; and, whereinthe protective membrane architecture comprises one or more materialsconfigured to provide a first membrane surface chemically compatiblewith the alkali metal anode in contact with the anode, and a secondmembrane surface substantially impervious to and chemically compatiblewith the components of the cathode compartment.
 2. The cell of claim 1wherein the hydrogel structure comprises a hydrophilic polymer.
 3. Thecell of claim 1 wherein the hydrogel structure absorbs moisture from theambient air during device operation.
 4. The cell of claim 1 wherein thehydrogel structure comprises solid phase active salts.
 5. The cell ofclaim 1 wherein the hydrogel structure comprises aqueous catholyte. 6.The cell of claim 1 wherein the hydrogel structure accommodates solidproducts of discharge.
 7. The cell of claim 1 wherein the hydrogelstructure retains catholyte and swells via absorption of moisture fromthe ambient air.
 8. The cell of claim 1 wherein the hydrogel structureis or comprises a natural polymer.
 9. The cell of claim 1 wherein asolid phase component of the hydrogel structure is a synthetic polymer.10. The cell of claim 1 wherein the hydrogel structure is a physical ora chemical gel.
 11. The cell of claim 1 wherein the hydrogel structureis a multicomponent hydrogel structure of two or more hydrogelcompositions.
 12. The cell of claim 1 wherein the hydrogel structurefurther comprises hydrogel contact layers having a composition differentthan that of the bulk hydrogel.
 13. The cell of claim 1 wherein thereservoir is a hydrogel.
 14. The cell of claim 1 wherein the hydrogelstructure forms in-situ as water from the ambient air is absorbed intothe cathode compartment.
 15. The cell of claim 1 wherein the hydrogelstructure retains catholyte and swells upon discharge with watermolecules generated via the cell reaction.
 16. The cell of claim 1,wherein the hydrogel structure is a multi-layer, comprising a firsthydrogel layer adjacent the anode and a different second hydrogel layeradjacent the cathode.
 17. A method of making a battery cell comprising:assembling the cell of claim 1, wherein the cathode is fabricatedoutside of the cell prior to cell assembly.